Marine Reptiles of the Mesozoic

Their ancestors were land-dwelling reptiles from the Permian and Triassic Periods. Except for the marine crocodiles, they were not closely related to dinosaurs. Some marine reptiles trace their ancestry to the roots of the reptilian evolutionary tree, while others were close relatives of lizards and snakes. Like today's turtles and crocodiles, they would hold their breath while under water.

Reptiles first dominated life on land about 300 million years ago, during the late Carboniferous Period. By the end of the Triassic Period, dinosaurs had begun their long stand as masters of the land, while their flying relatives, the pterosaurs, had taken control of the sky. Another group of reptiles took to the sea, where fishes, squids, shelled mollusks, and other sea creatures had long been thriving, nearly unchecked by any significantly larger predatory animals. Fish traveled in huge, silvery schools. Ammonites—soft cephalopod cousins of the squid that lived in coiled shells—propelled themselves through the ocean in large groups, feeding on fishes and other smaller creatures. It is not surprising that some of the highly adaptable reptiles gradually took to the oceans to get their share of this rich bounty of food. Some reptiles gradually returned to the seas sometime in the Permian Period. By the Late Triassic and Jurassic, many surprisingly varied forms of predatory marine reptiles had evolved. Marine reptiles are grouped in the following categories: Ichthyosaurs. Tuna- or dolphinlike in appearance, but unrelated to them, ichthyosaurs were one of the most common types of marine reptiles of the Triassic and Jurassic. Ichthyosaurs were the first important reptile group to successfully adapt to marine life. They have been found in many parts of the world and thrived throughout most of the Age of Dinosaurs, though they declined in the Cretaceous Period and likely became extinct before its end. Some were as small as 6 feet (1.8 m) long while others included what was probably the largest ancient marine reptile of all—an ichthyo-saur measuring a whopping 77 feet (23 m) long! That's longer than a sperm whale.

Placodonts. These were small to medium-sized, bottom-feeding reptiles that resembled armor-plated walruses. One genus, Henodus, bore an astonishingly close resemblance to turtles, though it was not related to them. Placodonts were extinct by the end of the Triassic Period.

Nothosaurs. Streamlined, swift, seal-like predators of the Triassic Period, the nothosaurs and pistosaurs had long necks and tails as well as powerful paddles for swimming. They were probably related to the plesiosaurs, which evolved during the Late Triassic.

Plesiosaurs. These were long- and short-necked open-ocean predators. The long-necked varieties could maneuver quickly to catch fish in their jaws. The short-necked varieties had gaping jaws and were often the top predators in their underwater world. Some of the largest short-necked plesiosaurs, such as Kronosaurus and Liopleurodon, preyed on any size of animal, including giant sharks.

Marine crocodiles. Several kinds of ocean-going crocodiles evolved during the Mesozoic Era. All are extinct but distantly related to the crocodiles that still exist.

Mosasaurs. These ancient predators had short necks, powerful jaws typically lined with cone-shaped teeth, and long, muscular tails to proper them through the water. Some grew to be more than 33 feet (10 m) long. They have been likened to large, underwater monitor lizards and are thought to be closely related both to these lizards and to snakes.

Marine turtles. These were the first true seagoing turtles. Although they are all extinct, they are related to modern sea turtles.

Marine reptiles were discovered in Europe long before the word dinosaur was coined. The skull of Mosasaurus ("Meuse River lizard") was discovered in 1780 deep inside a limestone mine in Maastrictht, Netherlands. By 1842, when British paleontologist Sir Richard Owen (1804-1892) coined the word dinosaur, examples of all the major marine reptile groups had already been described, including mosasaurs, plesiosaurs, ichthyosaurs, nothosaurs, plac-odonts, marine crocodiles, and marine turtles. In many ways, the study of ancient marine reptiles helped form the foundation of the scientific discipline study that came to be known as vertebrate paleontology—the study of extinct animals with backbones. This chapter provides an overview of the major clades of extinct marine reptiles that lived during the Mesozoic Era.